Halley’s Comet is the most famous periodic comet, returning to the inner solar system approximately every 76 years. Its predictable appearances have fascinated observers for millennia, with records dating back to 240 BC. Scientists study its fundamental makeup because comets retain materials largely unchanged since the solar system’s formation.
The Physical Structure of the Comet
The main body of Halley’s Comet is the nucleus, the solid core where its materials are stored. Data from the 1986 Giotto probe flyby revealed the nucleus to be an elongated, irregular shape, often compared to a potato. This central mass measures roughly 15 kilometers long and 8 kilometers wide.
Scientists once described comets as “dirty snowballs,” but observations refined this concept to a “snowy dirtball” with a dark, crusty surface. The nucleus has an extremely low mean density, estimated at only about 0.55 grams per cubic centimeter. This suggests the material is loosely aggregated, like a cosmic rubble pile. The surface is surprisingly dark, reflecting only about four percent of incident sunlight, making it one of the darkest objects in the solar system.
The Chemical Ingredients: Ice, Dust, and Organics
The composition of Halley’s Comet is a mix of frozen gases, silicate dust, and complex carbon-based molecules, representing a sample of the early solar nebula. The volatile components are predominantly ices, with water ice being the most abundant. When the comet approaches the Sun, the ejected gas is about 80% water vapor.
The remaining volatile materials include frozen carbon monoxide (around 17%) and carbon dioxide (3 to 4%). Traces of other frozen compounds, such as methane and ammonia, are also present, often trapped within the dominant water ice. These volatile ices are mixed with non-volatile dust particles that resemble the silicates found in terrestrial rocks.
The nucleus also contains significant organic material—complex molecules rich in carbon, hydrogen, oxygen, and nitrogen (CHON particles). These compounds are thought to be responsible for the nucleus’s low reflectivity, forming a dark, sooty crust over the surface. This insulating layer prevents the ice underneath from sublimating, meaning only a small fraction of the nucleus is active at any given time.
How Halley’s Comet Reveals Its Composition
The spectacular visual phenomena associated with Halley’s Comet are a direct consequence of its chemical makeup interacting with solar radiation. As the comet nears the Sun, intense solar heat causes the volatile ices to skip the liquid phase and turn directly into gas, a process called sublimation. This gas rapidly expands away from the nucleus, carrying along fine dust particles trapped within the ice.
The resulting enormous cloud of gas and dust surrounding the nucleus is the coma, which forms the comet’s temporary atmosphere. This outflowing material is then shaped by the solar wind and radiation pressure into the two distinct tails. Observations of the emitted light from the coma and tails allow scientists to confirm the presence of water, carbon compounds, and dust.
The dust tail is composed of the rocky silicate and organic particles released from the nucleus, pushed away from the Sun by the pressure of sunlight. This tail tends to be broad, curving, and slightly yellowish as it reflects the sunlight. The second, straighter feature is the ion or gas tail, formed when solar ultraviolet radiation ionizes the volatile gases, creating charged particles. These ions are then quickly swept straight away from the Sun by the solar wind, revealing the comet’s gaseous composition.